In the present invention, media signals include an audio signal and a video signal. And, the audio signal is explained as an example in the following description.
Currently, 2-channel signal is most frequently generated and user. Yet, the use of multi-channel signals gradually increases. In the /following description, an audio signal including at least three channels is called a multi-channel signal to be discriminated from the 2-channel signal. In general, an encoder compresses a multi-channel signal into a mono- or stereo-type downmix signal instead of compressing channels of the multi-channel signal individually. A downmixing unit of the encoder extracts spatial information by downmixing multi-channels. The encoder transfers the compressed downmix signal and the spatial information to a decoder or stores them in a storage medium. The spatial information is used in reconstructing an original multi-channel signal from the compressed downmix signal. In case of using an encoder and decoder for 2-channel signal compression and reconstruction, the encoder generates a downmix signal and spatial information from a 2-channel signal and then transfers a bitstream including them to the decoder. The decoder upmixes the transferred bitstream to generate the original 2-channel signal. In case that the encoder and decoder are used for compression and reconstruction of a multi-channel signal, the encoder generates a downmix signal and spatial information from the multi-channel signal and then transfers a bitstream including the downmix signal and spatial information to the decoder. The decoder then upmixes the transferred bitstream to generate the original multi-channel signal.